Testosterone! The man-molecule! You've probably read by now about the study published in this month's PNAS and reported on by the New York Times that found a link between fatherhood and levels of testosterone. Given how much we identify testosterone with manliness and virility, the results of the study could be seen by many as either a good or bad thing. The news reports are all working hard to spin it as a positive (via the "men evolved to be caring nurturers," interpretation), while acknowledging that some men will look at the loss of testosterone as an unequivocal negative.

First of all, let's review what testosterone actually is (and isn't). Testosterone is a hormone that, like most hormones, has many functions. Although we think of is as a "male," hormone it is present in both men and women. Men have higher rates, on average, but women tend to be more sensitive to the hormone. In men, testosterone is produced in the testes, although production levels are controlled by the pituitary gland. In utero and at puberty testosterone plays a large role in the development of what we think of as typically male characteristics. In adults, testosterone is involved in sperm development and muscle growth and may have an effect on libido, but it also plays a role in decidedly unsexy bodily functions, like platelet aggregation (an important part of blood clotting) and maintaining bone density. The thing about hormones that we tend to forget, though, is that they generally don't act alone. Sperm production, for instance, is regulated not just testosterone, but also follicle stimulating hormone, inhibin and gonadotropin hormones and involves the hypothalamus, and pituitary gland on top of the testes (and those are just the parts we know about - the process is not completely understood).

In other words, saying that testosterone is responsible for any aspect of "maleness," is an extreme oversimplification and while the levels of a single hormone may have an effect on certain things, it's never as simple as "testosterone does X." Furthermore, while we may equate testosterone with virility more isn't necessarily better. Abnormally low levels of testosterone will cause negative effects such as loss of sperm production and shrinking of the testes, but too high levels will cause negative side effects such as...loss of sperm production and shrinking of the testes. The ideal level of testosterone (if such a thing could be said to exist) is really a moderate amount, although the range for healthy males is fairly broad. Testosterone levels vary from person to person and for a healthy male could fall anywhere between 350-1200 ng/dl. Those levels also tend to drop as men age.

Okay, now that we have that background: onto the research. The study involved 624 men from the Philippines who were involved in a longitudinal nutrition study. The men answered a questionnaire and gave saliva samples (measured at waking and before bed, because testosterone levels vary throughout the day) in both 2005 (when their mean age was 21.5 years +/- 0.3) and again in 2009 (when their mean age was 26 years +/- 0.3). This allowed the researchers to compare testosterone levels not only between men who were fathers and those who were not but also in the same man before and after becoming a father. The participants came from a region of the Philippines were fathers were generally involved in daily care of children. The researchers hypothesized that men with higher testosterone levels would be more likely for find partners and have children and that, conversely, men who spent significant time with their children would have lower testosterone levels. In other words, while high testosterone levels may enable men to start a family, those levels would in turn drop once that family came into being.

The men were divided into four groups. The men in Group 1 (n=83) were partnered and fathers at the beginning of the study, in 2005. Group 2 (n=257) were not partnered or fathers in either 2005 or 2009. Group 3 (n=46) became partnered between the two time-points of the study but did not have children, and Group 4 (n=162) became partnered and had children between 2005 and 2009. All of the men experienced a decline in testosterone levels between 2005 and 2009, but the decrease was most dramatic for Groups 3 and 4. Men in Group 3 experienced an average drop in testosterone levels of 10% for AM levels (those measured in the morning) and 32% for PM levels (those measured in the evening). New fathers, on the other hand, experienced a drop of 26% for the AM measurements and 34% for PM. Strangely, the men who experienced the least decline in testosterone were those who were partnered and fathers at the beginning of the study, although those men had lower testosterone levels, which may indicate that they had experienced a drop before the study began. Testosterone levels were also correlated with the age of the men's children and the amount of time they spent interacting with them. Fathers of newborns experienced a dramatic drop in testosterone levels, while those with older children experienced significantly less of a decrease. The results remained significant even when accounting for other factors such as lack of sleep (newborns aren't just cute, they're exhausting too). Testosterone levels also decreased more for men who spent a significant amount of time with their children, though the differences were less dramatic than those based on the children's age.

The study results are interesting, and the fact that they looked at changes in men over time (as opposed to just differences between men) makes it easier to control for external factors. The researchers are able to say, for example, that the lower testosterone levels of men who spent significant time with their children was not due to men with lower testosterone to begin with choosing to be more involved with childcare, since those men's initial testosterone levels were not on the low side (in fact the partnered men with children had higher initial testosterone levels than the unpartnered men). However, the small sample size and the fact that there are essentially only two data points per individual mean that the margin of error is fairly high and the evo-psych explanations for the data that many sites are reporting on are, as usual, stretching what can reasonably be concluded. Does having children lower men's testosterone? It would appear so, based on this study and previous ones, but whether this change is permanent and how it affects fathers in general is hard to say given how muddy the relationship between any individual hormone and physical and mental traits tends to be. Lowered testosterone will not necessarily make a man more nurturing, although it could improve his health (high testosterone levels correlate with numerous health issues) and make him less aggressive or prone to risky behaviors. Considerably more data is needed, however, to understand the relationship between fatherhood and testosterone levels.

(Headline provided by Ninny Father, so don't take it out on Dr. Pisaster. -- DR

Dr. Pisaster has a doctorate in biophysics, not actually anything sexy. She does however enjoy having sex, reading about sex, and talking about sex. Especially when she's had a little whiskey.